I'm trying to have a field on my master object (Opportunity, field is called AFPAK_Student_Names__c) reflect the a field on all detail objects (Student_Assignment__c) meeting a certain criteria (Student_Assignment__c.status = Active) at any given time. Kind of like a text roll-up summary field. 

This trigger works just fine to add student names to the field when I add new Student Assignments with Status Active. However, what if I delete one or change the status? I want the field to reflect the change. 

In that case, is there a way to use Apex to test if the text field contains certain text (Student Name from the now non-active Student Assignment) and then remove just that? 

trigger StudentAssignmentClassUpdate on Student_Assignment__c (after insert) { 

if (trigger.isInsert){
Map<Id,Id> AssignmentToClassMap = new Map<Id,Id>();
for(Student_Assignment__c A : trigger.new)
           AssignmentToClassMap.put(A.Class__c,A.Id);
 
List<Opportunity> OppsToUpdate = new List<Opportunity>();

      
    for (Opportunity TestOpp: [SELECT Id, AFPAK_Student_Names__c FROM Opportunity WHERE Id IN:  AssignmentToClassMap.keySet()])
    {
        Id TestOppId = AssignmentToClassMap.get(TestOpp.Id);
        Student_Assignment__c StudAss = trigger.newMap.get(TestOppId);
        
               
//CASE 1: if inserting a Student Assignment with status Active, add student name to AFPAK student names field        
if (StudAss.status__c=='Active'){
TestOpp.AFPAK_Student_Names__c =TestOpp.AFPAK_Student_Names__c+' '+[SELECT Name From Contact Where Id = :StudAss.Student_Name__c limit 1].Name;
OppsToUpdate.add(TestOpp);
}
}
if(OppsToUpdate != null && !OppsToUpdate.isEmpty())
Database.update(OppsToUpdate);
}
}